The bile pigments are metabolic products of haemin synthesis and degradation that, in addition to their physiological importance, are interesting as a class of molecules because of their structural mutability when interacting with light. These X-ray diffraction studies have shown, for a model compound, the photoisomerism about a double bond that is an essential mechanism in bilirubin elimination. Two molecules under study are model compounds for the chromophore of the protein, phytochrome. Current and future studies will pursue this avenue as well as explore the structural properties of related pyrrolic compounds. Other tools used in these studies are chemical (especially structural) information retrieval techniques and the display of molecular structures by means of interactive computer graphics.